1. Field of the Invention
The present invention relates to motor driving circuits, and particularly to a DC brushless motor having no position detecting mechanism separately provided.
2. Description of the Background Art
FIG. 21 is a circuit diagram of a driving circuit for driving a DC brushless motor having no position detecting mechanism provided. A motor M includes coils 51-53 connected in common to a middle point connected to a terminal 8A. These coils 51-53 are connected to a current control portion 300 through terminals 5A-7A. Transistors 1-6 included in the current control portion 300 are controlled by a 120.degree. switching circuit 109 to turn off and on, and the motor M receives three-phase current to operate.
The 120.degree. switching circuit 109 is controlled by a start circuit 110 and the motor M is activated. The frequency of a signal provided to the 120.degree. switching circuit 109 from the start circuit 110 is determined in capacitors 34 and 35.
The timing of applying an electric current to the coils 51-53 of the motor M is switched by the control of the 120.degree. switching circuit 109. A position detecting portion 200 provides position detection signals D1-D3 to the 120.degree. switching circuit 109. In a DC brushless motor which has no position detection mechanism, the counterelectromotive force of the motor is used for position detection to continuously run the motor M. Accordingly, the counterelectromotive force of the motor M is inputted to the position detection portion 200 through terminals 1A-4A. That is, the terminals 1A-4A and the terminals 5A-8A are connected to one another. In the position detection portion 200, comparators 201-203 compare potential at the middle point and potential at each of the terminals 5A-7A to output the position detection signals D1-D3, respectively.
FIG. 22 is a timing chart showing driving of the motor M. Now, if considering the case in which there is no filter formed of a capacitor 36 and a resistance 39 at an input end of the comparator 201, a voltage R1 at the coil 51 varies as shown by the broken line in FIG. 22. The position detection signal D1 detects a zero-cross point of the voltage R1 and presents the waveform illustrated by the broken line. Similarly, at input ends of the comparators 202 and 203, assuming that there is no filter, the position detection signals D2 and D3 present the waveforms shown by the broken lines. The 120.degree. switching circuit 109 synthesizes these signals D1-D3 to output a driving signal K1 (the broken line) for controlling the transistor 1 in the current control portion 300.
To rotate the motor M efficiently, however, the period in which the driving signal K1 for driving the transistor 1 is activated must be delayed by 30.degree.. Accordingly, filters including capacitor 36 and a resistor 39, a capacitor 37 and a resistor 40, and a capacitor 38 and a resistor 41, respectively, are provided at input ends of the comparators 201-203. These filters are designed so that the waveforms of the position detection signals D1-D3 are delayed by 30.degree. (the solid lines) as compared with those in the case in which there is no filters provided (the broken lines).
Receiving the position detection signals D1-D3 shown by the solid lines which are obtained in this way, the 120.degree. switching circuit 109 outputs the driving signal K1 shown by the solid line to drive the transistor 1. Similarly, the switching circuit 109 outputs driving signals for driving the transistors 2-6. As a result, the voltage R1 presents the waveform shown by the solid line.
In the conventional driving circuit for driving the DC brushless motor having no position detection mechanism, the driving signal for driving the motor is delayed by 30.degree. by means of a filter including the CR. Accordingly, it can run the motor efficiently only at a particular number of rotation, but it has a problem that it can not enable efficient operation at other rotation numbers.